As electrolyte solutions for electrochemical devices such as electrical double layer capacitors, those obtained by dissolving quaternary ammonium salts or the like in organic solvents such as cyclic carbonates (e.g., propylene carbonate) or nitrile compounds (see Patent Literature 1) are often used. Impurities contained in such electrolyte solutions are known to possibly cause an adverse effect on characteristics of electrochemical devices by, for example, reducing the voltage resistance or capacity of the electrochemical devices. To solve the problem, various studies have been made for reducing impurities contained in electrolyte solutions.
One example disclosed is an electrical double layer capacitor including a nonaqueous electrolyte solution prepared by dissolving a quaternary ammonium salt in a cyclic carbonate, the nonaqueous electrolyte solution containing 30 ppm or less of glycols, 30 ppm or less of monovalent alcohols, and less than 20 ppm of tertiary amines as impurities (see Patent Literature 2).
Another example disclosed is an electrolyte solution for electrochemical capacitors prepared by dissolving an electrolyte salt mainly containing a quaternary ammonium salt in a nonaqueous solvent, wherein the concentration of tertiary amines and tertiary ammonium salts in total is 2 mmol/kg or less (see Patent Literature 3).